At the Intersection of Biomass & Urban Ecology

In spring 2015, Minnesota’s capital city of nearly 300,000 people, Saint Paul, launched what it calls the EcoDistrict, a four-block section of downtown along the Mississippi Riverfront designed to educate people on the benefits of environmentalism and showcase Saint Paul’s energy innovation. The EcoDistrict features multiple solar installations, district energy, heat recovery, combined heat and power (CHP), composting programs, and other renewable energy and advanced technology solutions.

“With over half of the world’s population currently living in urban areas, cities are critical players in the fight to address climate change,” says Chris Coleman, Saint Paul’s mayor since 2006. “We need to engage the public in a meaningful way, and to create hands-on tools for people to explore how companies and organizations within our city are advancing ideas and solutions.”

According to the city, the EcoDistrict is part of a national effort to focus urban planning on sustainable development and to create places within a city in which environmental principles are integrated into spaces where people live, work and play. EcoDistrict partners include the city of Saint Paul, District Energy St. Paul, the Science Museum of Minnesota, Saint Paul RiverCentre and Xcel Energy Center, and Visit Saint Paul.

In the EcoDistrict, multiple solar photovoltaic (PV) collectors contribute electricity to the Science Museum, a parking ramp, an electric vehicle charging station, and can also send electrons back to the grid for use around the city. PV solar panels are different from solar thermal technology, which converts heat from the sun into hot water for thermal energy. For solar thermal, a pump pushes water through pipes in the solar panels, and the hot water returns to inside the building. The water flows in a continuous loop, getting heated, distributed and used, and then the lower-temperature water returns to the rooftop panels to be heated back up. The installation on the Saint Paul RiverCentre generates hot water for the building, and the extra hot water is sent into a citywide grid of pipes for use in other buildings—part of the district energy system.

District Energy St. PaulAccording to a 2012 paper by the American Council for an Energy-Efficient Economy, more than 5,800 district energy systems operating in the U.S. today serve roughly 7 percent of commercial buildings, downtown districts, campuses, military bases, research facilities, and even some residential locations.

Launched as a demo project in 1983, DESP—a public-private partnership between the city of Saint Paul, the state of Minnesota, U.S. DOE, and the downtown business community—was the city’s response to the energy crises of the 1970s. DESP is said to have been built from the vision of Saint Paul Mayor George Latimer, who served as mayor 1976-’90. Under Latimer’s leadership, the city lobbied state and federal governments for assistance in adopting technology developed in Europe that could solve the city’s heating problems. Using the expertise of Hans Nyman, DESP’s first president, the system was designed to be energy-efficient and fuel-flexible. The utility that eventually became DESP was originally established a century ago, and relied solely on coal. As the demo project launched in 1983, one of the first transitions was from steam to hot water, followed by incorporating natural gas into its coal-dominated fuel mix.

Nina Axelson, the vice president of public relations for DESP, says first-generation district energy systems relied on once-through steam, but recovery was difficult. “Second-generation systems recoup some energy from the condensate line,” which was an improvement, but later-generation systems utilize hot water, which she says enables a continuous loop. Water is sent out from the plant at 250 degrees Fahrenheit where it travels via underground piping to customer buildings. Heat exchangers help transfer the heat to the indoor environment as needed, and the water received back at the plant comes in at around 160 degrees, Axelson says.

Newer systems—third- and fourth-generation iterations—are even more efficient, as they send out lower-temperature water, since the often newly constructed buildings served are higher-efficiency, lower-energy structures. “These new buildings may not need 200-degree water because they are better insulated and have a better energy profile,” Axelson says. “High-efficiency buildings are designed so their energy demand is less, so production and distribution are programed in district energy systems to meet the lower temperature point needed.”

Today, DESP boasts the largest hot water district system in the U.S., serving more than 200 buildings with heating in downtown Saint Paul, plus 300 residential homes, townhomes and small residential public housing complexes, Axelson says, through 42 miles of supply and return piping. The system circulates 1 million gallons of water per hour, with a supply temperature of 250 degrees in the winter and 190 degrees in the summer. DESP heats nearly 32 million square feet of indoor space throughout Saint Paul.

In 1993, 10 years after the DESP’s heating system startup, the utility began offering cooling service to downtown buildings. Today, 60 percent of downtown is connected to a district cooling system. DESP produces chilled water and distributes it through underground pipes to buildings where the chilled water removes heat from the internal spaces, cooling the air. The heat removed from the buildings is captured in the return water and piped back to the plant for chilling. DESP serves more than 100 buildings with cooling in downtown Saint Paul through 13 miles of supply and return piping. DESP circulates 2.5 million gallons of chilled water per hour, with a supply temperature of 42 degrees. Chilled water is produced at night using off-peak electricity and stored in two large thermal storage tanks.

Axelson says DESP has an 80 percent market share for heating in Saint Paul’s central business district, and a 65 percent share for cooling. Xcel Energy is its main competitor, but it’s also a partner since DESP buys electricity from the utility and, through a power purchase agreement, sells much of the electricity generated at its CHP plant, named St. Paul Cogeneration, back to Xcel Energy.

In 1997, DESP began cofiring wood chips with coal, Axelson says, but “the most substantial shift was when we started up the cogeneration unit in 2003,” she adds. In 2003, DESP commissioned a CHP system fueled by urban wood waste. “The boiler was by Foster Wheeler and the turbine by GE-Thermodyn,” Axelson says, adding that Trigen-Cinergy Solutions was the original business partner to Ever-Green Energy for development and ownership of the St. Paul Cogeneration facility. “This ownership stake has since been acquired by DTE [Energy Resources through its subsidiary DTE St. Paul LLC],” she says. Ever-Green Energy serves as the operator and manager, and it oversaw design and construction of the facility.

A CHP system combusts fuel to create steam that pushes a turbine. While the leftover steam does not have enough energy to be effective in the turbine again, plenty of thermal energy remains and is captured for its heating value.

St. Paul Cogeneration produces about 65 megawatts (MW) of thermal energy for DESP and approximately 33 MW of electricity. Up to 25 MW of this biomass power and 153,000 MW-hours are supplied to the local electric utility—enough electric supply for up to 20,000 homes. The CHP system consumes about 800 to 1,000 tons of wood waste per day, averaging about 250,000 tons per year, says Jeff Guillemette, biomass fuel manager for Environmental Wood Supply, which supplies wood to St. Paul Cogeneration. Axelson says biomass provides 90 percent of the fuel to the cogen boiler. “The heat captured from the cogeneration process only makes up approximately 40 to 50 percent of our overall heating load,” she says. “The rest is sourced from natural gas boilers.” St. Paul Cogeneration states that natural gas is mixed with the wood chips in the boiler to increase the combustion temperature and add stability. According to DESP, St. Paul Cogeneration is the largest wood-fired CHP plant serving a district energy system in the U.S.

Biomass ProcurementEWS was formed by Ever-Green Energy and DTE St. Paul LLC as the biomass procurement arm of St. Paul Cogeneration, about a year after the CHP system went into operation. “They had issues with procuring enough wood locally,” Guillemette says. “That went on for more than a year, if not longer, and then the decision was made to take ownership of procuring wood and to get more aggressive with our own equipment to do offsite grinding. The processing yard was already in place, but the mechanism to procure wood locally wasn’t there. That’s when EWS was created. They built relationships and a network to capture the offsite grinding.”

Guillemette says while some of the regional urban wood waste in the Twin Cities of Minneapolis/Saint Paul was utilized for mulch and other purposes, much of it was truly a waste—going to construction and demolition landfills. “We work in a 50-mile radius with an offsite grinding crew to capture as much wood waste as we can,” he says, adding that EWS has built a network of 40 to 50 municipalities from which EWS routinely procures biomass. “We have a tree brush drop-off for residents,” Guillemette says. “At some locations, we grind once a year, but at others maybe we do three to four grinds a year. It depends on volume.”

Then there is a main yard, Guillemette says, where wood waste is collected from industrial or commercial companies that do tree removal, trimming, land clearing and refuse disposal. “In addition, we work in an area outside the 50-mile radius with subcontractors, some of the larger cities there, and we charge a mobilization fee to make that more cost-effective for us,” he says. That covers the cost to move EWS’s equipment to the site and back to the wood yard. “It’s expensive to move tub grinders and wheel loaders 75 miles away,” Guillemette says. “The city or private wood yard pay the fees. The wood is usually a ball of yarn when we show up—a big pile. They would usually pay a tipping fee to get rid of it. They all have options other than us, but they’re all more expensive than our mobilization fee.”

Finally, a smaller percentage of waste wood EWS captures for St. Paul Cogeneration comes from loggers as forest residues. “We accept that material as well,” Guillemette says, “but our main focus is to capture as much urban wood waste as we can.”

After the material is ground, it is transported to the wood yard and piled. After about two weeks, the wood is ground or screened once more before being loaded onto trailers by wheel loaders at one of three ramps throughout the wood yard, and trucked three miles to St. Paul Cogeneration. “There’s nothing too fancy at the wood yard,” Guillemette says. “There’s no dryer, no covered inventory, it’s all open to the elements. The moisture content varies throughout the year, but on average it is 35 percent.” He adds that the boilers are robust enough to handle variation in moisture content.

The trucks drop off the ground wood waste using walking floor trailers, and the material is conveyed into one of two silos that are filled throughout the day. The plant is continuously drawing from the silos to combust roughly 1,000 tons a day.

The FutureWhile DESP doesn’t have a hard transition date as to when natural gas was introduced into the fuel mix, Guillemette says from 1983 to 2003, coal was used 12 months out of the year. “Now we’re only using coal for four to five months of the year,” he says. Axelson adds that DESP’s coal use is down to below 10 percent of the utility’s previous permitted level. “The only coal we’ve used this past winter was on test runs,” she says. “Last winter we used less coal than any previous year. We’re on the home stretch in getting off coal.”

In late 2015, DESP announced plans to completely eliminate the use of coal for its heating system after the 2020-’21 season, further reducing its carbon dioxide emissions by 27 percent, or 21,000 tons. The fuel transition is not mandated, DESP states, “but driven by efforts to reduce greenhouse gas (GHG) emissions, increase system resilience, and deliver a value proposition that is in line with customer demands.”

Ken Smith, CEO and president of DESP, says, “It is now time to phase out these assets and use this opportunity to modernize and diversify energy sources. Cities are the largest contributor to energy use and GHG emissions. It is our responsibility as a community energy system to find innovative solutions that lessen our impact, improve local resilience, and help create a city prepared for the future. The elimination of coal is an important step in the evolution of our business, and we are already looking ahead to our next opportunity to save energy and reduce climate impacts.” Mayor Coleman adds, “In the face of a changing climate, the city of Saint Paul relies on its service partners to continue improving their environmental performance and deliver the next great energy solutions that reduce our carbon footprint and make us more resilient to changes in our environment and our economy.”

To eliminate coal entirely, DESP plans for additional diversification to incorporate greater efficiencies, emerging technologies, and more renewable energy sources. Primary opportunities include incorporation of additional waste heat from the CHP system and waste heat capture from the city’s systems.

“I don’t have specifics,” Axelson says when asked exactly how DESP will eliminate coal. “It’s an ongoing look at optimization of the plant. There’s a lot of moving parts. The cogen unit is more fixed, but we’re looking at optimizing elements of that as we look at our heating assets. There’s a big range—cooling tower recovery vs. flue gas recovery, lower-temperature sources, or the ability to put a low-temperature loop on the system along the river, new construction developments. Right now, everything’s in the hopper.” As for what companies DESP is working with in its latest efficiency and optimization efforts, Axelson says, “All of our current planning is internal. Once specifics are determined, we will integrate partners.”

With a new U.S. president in office and what seems an entire administration that, at best, is skeptical of climate change, Axelson says DESP remains committed to providing reliable energy from renewable, efficient and cost-effective sources. “We will not be changing our commitment to sunset coal, because this decision is the right option for our customers and the community,” she explains. “There is lots of excitement in the market to develop more wind and solar, which are important but variable sources. We hope policymakers and investors continue to see the importance of adding renewable sources to the baseload profile, which is a great fit for biomass, biomethane, combined-heat-and-power, and geoexchange projects that use the latent energy in sewer systems. We need to keep looking at readily available, localized resources that can bolster our energy systems as well as support development and jobs for the local economy. There is room for multiple solutions to solve our complex energy challenges and the more flexible and integrated we are, the more consumers and the environment will benefit.”